Objective: To investigate the expression of both thymie regulatory T cells (CD4+CD25+Foxp3++cells, Treg) and thymic stromal lymphopoietin (TSLP) in thymomas accompanying myasthenia gravis. Methods: We used immunohistochemistry and real-time reverse trancription polymerase chain reaction (real-time RT-PCR) techniques to determine Foxp3+ Treg counts and the expression levels of Foxp3 mRNA and TSLP mRNA in thymomas of 23 MG patients and thymuses of 4 healthy controls. Results: The CD4+ Foxp3+ nTreg (natural regulatory T cells) counts in thymomas were significantly lower than those in normal thymuses (P<0.01), and the expression levels of Foxp3 mRNA and TSLP mRNA were also lower in thymomas(P<0.01). Among the thymoma types, type B1 thymoma had the highest Foxp3+ nTreg count and standard values of Foxp3 mRNA and TSLP mRNA. There was a strong positive correlation between the mRNA transcriptional levels of Foxp3 and TSLP. Conclusion: The insufficient expression of Foxp3 in thymoma, which may be caused by decreased transcription of TSLP, may result in the reduction of Tregs and cause autoimmune disorders.